This invention relates to the production of emulsion-polymerized butadiene rubber in dry, free-flowing, finely-divided particulate or powder form and is directed to an improved and simplified method for obtaining such rubber in such form directly from the latex resulting from the emulsion polymerization. The method involves the use of special and critical techniques in preparing and coagulating the latex, preferably followed by special coagulum-treating procedures.
As is well known in the art, emulsion-polymerized butadiene rubber, such as the rubbery copolymers of butadiene-1,3 and acrylonitrile known as "nitrile butadiene rubber" or "NBR", is made by first polymerizing the monomers in aqueous emulsion in the essential presence of an emulsifying agent and a polymerization initiator (sometimes called a polymerization catalyst) together normally and desirably with various other additives affecting the polymerization or the properties of the rubber, to form an aqueous dispersion or liquid latex in which the solid dispersed phase is made up of microscopic rubber particles. The rubber is then obtained in dry solid form by coagulating the latex to produce a mass of coagulum, generally in the form of large crumbs or lumps (due to easy and rapid agglomeration or sticking together of the soft rubber particles as coagulation occurs), which is separated from the aqueous phase, dried and formed into sheets or bales. The baled rubber is supplied to rubber goods manufacturers where it is processed, compounded and vulcanized into finished rubber products.
Producers of NBR have recognized that processing operations in the rubber goods factory can often be facilitated and made less expensive and energy intensive by supply of the rubber in powder form. Consequently, several powder grades of NBR, produced by mechanically pulverizing or grinding the dry sheet rubber as produced in the usual manner, have been offered to the trade. Such powdered rubber is composed of discrete particles of "powder size" by which is meant a size in the range of about 0.3 to about 4 millimeters (mm) in greatest dimension (diameter) with average particle size (defined as the size at which 50 weight percent of the particles are smaller) of 0.5 to 2.0 mm. The rubber in this powder form can be processed and compounded by powder handling techniques and vulcanized to form finished rubber products of comparable properties, given a predetermined compounding recipe, as when starting with baled rubber and processing and compounding in the more conventional manner. However, the extra grinding step in the rubber production process proceeds slowly and consumes considerable energy requiring a premium price for the powder variety, which offsets to a substantial degree the savings in processing costs to the rubber products manufacturer. As a result such powdered rubbers have had only limited commercial acceptance and success.
Another known way to produce emulsion-polymerized butadiene rubber in powdered form is by spray drying the latex. This procedure is also slow and laborious and the powdered product contains all the solid ingredients present in the latex, including the water-soluble emulsifier used in the emulsion polymerization, and hence, is extremely water sensitive and water swellable. Spray dried powders are also often too fine to be handled except for dissolving in solvents to form adhesives, for example. These factors as well as other problems involved in the spray drying process have dictated against spray drying as a means for producing powdered butadiene rubber for general use in manufacture of finished rubber products.
There have also been proposals for producing powdered butadiene rubbers directly from a latex thereof by processes involving coagulation. Patents describing such processes include British Pat. Nos. 924,699; 924,700; 1,008,659 and 1,113,348; German Pat. Nos. 1,148,067; 1,204,404; 1,204,405 and 1,215,918; Japanese Pat. Nos. 54 (1979) 7448 and 7449 and U.S. Pat. Nos. 3,494,784; 3,573,227; 3,813,259; 4,119,759 and 4,269,740. In these processes, procedures are not as simple as desired and various additives are used in the coagulation such as various forms of silica and various harder-than-rubber synthetic resins and polymers which function to "partition" or "encapsulate" the soft rubber particles as coagulation occurs. These additives remain in the powdered rubber which would by supplied to rubber products manufacturers and since they are dissimilar to rubber and to conventional rubber compounding ingredients, their presence is not generally desired.
The background of the invention gives no indication of how butadiene rubber could be easily produced in dry powder form without substantially increasing energy usage and without substantially affecting its utility by manufacturers of rubber products.